Apparatus for passing stock into a headbox of a paper machine or equivalent

ABSTRACT

The invention relates to an apparatus for passing stock to a headbox ( 100 ) of a paper machine or equivalent. The apparatus comprises a deaeration tank ( 11 ) which is provided with vacuum by means of a vacuum pump (P o ) or another device. The deaeration tank ( 11 ) comprises an inlet duct ( 10 ) through which it is supplied with wire water. The deaeration tank ( 1 ) comprises a discharge duct ( 12 ) and, at the discharge end thereof, an overflow ( 14 ) for the wire water in the discharge duct ( 12 ). Said overflow ( 14 ) of the discharge duct ( 12 ) is located below the deaeration tank ( 11 ) and opens to a free air space, and that the discharge duct ( 12 ) includes a branch duct ( 15   a   1 ) for a flow which is passed to the headbox ( 100 ).

[0001] The invention relates to an apparatus for passing stock to aheadbox of a paper machine or equivalent.

[0002] When a deaeration tank is situated between centrifugal cleaningand a headbox feed pump in the short circulation, it should act not onlyas a means of removing air but also as a pressure equalizer before theheadbox feed pump. However, the deaeration tank does not guarantee aconstant pressure since, in the pressure conditions of the deaerationtank, air bubbles take a large part of its liquid volume. For example,5% of air takes 50% of the liquid volume at a pressure of 0.1 atm. Thefoaming of air bubbles in the deaeration tank is unstable. The overflowfrom the deaeration tank evens out the surface but it pays no attentionto whether there is air or gas in the liquid space of the deaerationtank. For this reason, the pressure after the deaeration tank varies.The pressure variations are transmitted to the headbox. The pressurecontrol of the headbox attempts to maintain a uniform pressure in theapproach pipe, but it affects the level of the surface in the deaerationtank, intensifying the disturbance. Mere removal of the overflow fromthe deaeration tank is not enough because the pressure variation turnsinto consistency variation.

[0003] In accordance with the invention, the deaeration tank can beaccomplished without an overflow if it is in hydraulic connection withan overflow surface in the cellar. For example, the lock water part in acirculation water tank can act as an overflow surface if the deaerationtank is arranged to treat wire water in the short circulation. It isadvisable to build the overflow so that it is wide. When shortcirculation dilutions, among other things, for the headbox feed and fanpumps are taken from a connecting pipe, the pressure of the shortcirculation remains constant, because it is determined by a stableoverflow at normal pressure, in which there is no problem caused bybubbling of air. In controlling the vacuum in the deaeration tank, turboblowers may be more suitable than vacuum pumps. The process is simplerthan before, consumes less energy and requires a considerably smallerprocess volume. When the flow of wire water increases, the speeds in thecyclone and in the spray tubes of the deaeration tank increase, whichincreases deaeration capacity.

[0004] Thus, in accordance with the invention, the deaeration tank isprovided with an overflow which is disposed at the distance of thestatic height difference of the liquid column required by vacuum fromthe deaeration tank, for example, in the cellar space in the papermachine hall. The height difference between the liquid surface of thedeaeration tank and the overflow surface is advantageously in a range of5 to 10 m.

[0005] The back-pressure required by the pump is about 3-4 m counted asa static water column depending on the pump. The level of the overflowsurface need not be controlled, i.e. it may be fixed, so that it isabout 8 m below the surface of the liquid in the deaeration tank (8 mcorresponds to a vacuum of 80 kPa in the deaeration tank). In thatconnection, it must be possible to adjust the vacuum level in thedeaeration tank. The deaeration tank would then be located about 13 mabove the cellar level, i.e. about 4 m above the machine level. Thepressure loss in the spray feeding of the deaeration tank is about 3-5m. When the pressure loss of the deaeration tank is added to theback-pressure of the pump, a level slightly lower than the machine levelis achieved, which means that the waters coming from the wire can betreated with the system in question without additional pumping. Thediameter of the distributor pipe is in a range of 0.3 to 2.0 m and thetime in which the liquid flows from the deaeration tank to the biggestsite of use is less than 2 minutes, in practice about 5 seconds.

[0006] Thus, the invention employs a deaeration tank which has nooverflow, while, in accordance with the invention, the discharge duct ofthe deaeration tank comprises an overflow. The overflow is preferablydisposed in a wire water tank or equivalent. In accordance with theinvention, a branch duct/branch ducts is/are arranged to lead from theduct between the overflow of said discharge duct and the deaeration tankto a headbox, preferably to the suction side of a headbox feed pump.

[0007] The apparatus according to the invention provides a stock flow,which is uniform both in pressure and in consistency, to the suctionside of the headbox feed pump and further to the headbox.

[0008] The apparatus for feeding stock to a headbox of a paper machineor equivalent is characterized by the disclosure in the claims.

[0009] The paper machine or equivalent is understood to mean printingpaper, board, and soft tissue machines.

[0010] In the following, the invention will be described with referenceto some advantageous embodiments of the invention illustrated in thefigures of the appended drawings, to which the invention is, however,not meant to be exclusively confined.

[0011]FIG. 1A shows a first advantageous embodiment of the invention. Inthe embodiment of the figure, a discharge duct of a deaeration tank isarranged to lead into connection with a tank such that the end of thedischarge duct opens centrally in the tank and forms an overflow.

[0012]FIG. 1B shows an embodiment of the invention comprising adischarge duct which is connected directly to a tank which comprises anoverflow.

[0013]FIG. 1C shows an embodiment of the invention including a tankwhich comprises two overflows, i.e. an overflow for a discharge ductand, in addition, an overflow for the surface level of the stock in thetank.

[0014]FIG. 1D shows an additional embodiment of the invention.

[0015]FIG. 1E is an axonometric illustration of the short circulation ofstock in accordance with the invention in connection with a headbox of apaper machine, as well as of a centrifugal cleaning plant.

[0016]FIG. 1A shows a first advantageous embodiment of the inventionwhich relates to a stock feed system of a headbox in a paper machine orequivalent and to its short circulation. Wire water is passed along aduct 10 into a deaeration tank 11, which has a discharge duct 12 forwire water from which air has been removed, and which discharge duct 12comprises at its end an overflow 14, in which connection the heightdifference H₁ between the level T₂ of the overflow surface 14′ of saidoverflow and the surface T₁ of the wire water in the deaeration tank 11is in a range of 5 to 10 m. The deaeration tank 11 comprises for wirewater an inside tank space D, which comprises a vacuum space D′ abovethe liquid, into which space vacuum is drawn through a duct f by meansof a vacuum pump P₀, an exhaust pump or another device and, at the sametime, air is removed from the wire water. By the wire water is meantwater that is removed from a paper web on the paper machine.

[0017] As shown in FIG. 1A, the discharge duct 12 is arranged to leadinto connection with a tank 13 through its wall and upwards in the tank.The discharge duct comprises the overflow 14 which is located centrallyin the tank, for example, in a wire water tank 13. Branch ducts 15 a ₃,15 a ₂, 15 a ₁, open into the discharge duct 12 of the deaeration tank,wire water and thick stock m being passed through the duct 15 a ₂ ofsaid ducts first to the suction side of a feed pump P₂ and further to acentrifugal cleaning plant 200, from which the stock mixture is passedthrough a duct 15 a ₂′ to a mixing point of the wire water passed fromthe branch duct 15 a ₁ of the discharge duct 12, at which mixing pointthe stock mixture is diluted to a headbox consistency and passed to thesuction side of a headbox feed pump P₁ and further to an inlet header Jof a headbox 100. Dilution water is passed through the branch duct 15 a₃ to the thickness profiling system of the headbox of the paper machine.Advantageously, the connection points of the branch ducts are located inthe discharge duct 12 close to the overflow 14 and, in the heightdirection, below the overflow 14.

[0018] The discharge duct 12 can thus in itself comprise an overflow 14,i.e. the lower end 12′ of the discharge duct 12 is placed, as shown inFIG. 1A, centrally inside the wire water tank 13 and said end opensupwards to normal atmospheric pressure in the tank 13, in whichconnection a constant pressure prevails in the discharge duct 12. Theend result, then, is the same as in the preceding case, .i.e. a constantpressure prevails in the discharge duct 12. Wire water/stock is passedby means of a pump P₄ to the deaeration tank 11. Wire water is passedthrough a duct 17 of the tank 13.

[0019]FIG. 1B shows an embodiment in which a discharge duct 12 of adeaeration tank is connected to the tank 13 itself, which comprises anoverflow 14. The overflow 14 is formed such that the discharge duct 12opens into the tank 13 from its side surface 13′, in which case the tank13 itself comprises the overflow 14 in its connection. The heightdifference between the overflow 14 and the surface level T of the stockin the deaeration tank is designated by H₁ in the figure. H₁ is in arange of 5 to 10 m.

[0020]FIG. 1C shows an embodiment in which a tank 13 comprises twooverflows, i.e. an overflow 14 of a discharge duct 12 and, in addition,an overflow 140 which regulates the surface level of the stock in thetank 13 and, thus, the pressure prevailing in the tank. Wire water iscirculated from the tank 13 by means of a pump P₄ along a duct 10 to adeaeration tank 11, and wire water from which air has been removed ispassed along the discharge duct 12 into the tank 13. The discharge duct12 of the deaeration tank 11 is provided with a branch duct 15 a ₁ for aheadbox feed pump P₁, from which stock is passed further to an inletheader J of a headbox 100, and with a branch duct 15 a ₂ for a feed pumpP₂, from which stock is passed to a centrifugal cleaning plant 200 andfurther to the headbox 100 through a passage to the branch duct 15 a ₁.

[0021]FIG. 1D shows an embodiment of the invention in which an overflow14 of a duct 12 is formed such that the discharge duct 12 opens into atank 13 from its side surface. The tank 13 comprises the overflow 14 forthe liquid surface. Wire water from which air has been removed is passedthrough the discharge duct 12 to the tank 13. Wire water is pumped bymeans of a pump P₄ along a duct 10 into a deaeration tank 11, which islocated in an elevated position with respect to the tank 13. Thedischarge duct 13 comprises, in the vicinity of the overflow 14 but at alower level, branch ducts 15 a ₁, 15 a ₂, which comprise pumps P₁, P₂,in which connection stock is passed through the branch duct 15 a, to aheadbox and through the branch duct 15 a ₂ to a centrifugal cleaningplant 200 and further to the branch duct 15 a ₁. Virgin stock m is addedto the branch duct 15 a ₂. Wire water is passed

[0022] to the upper part of the tank 13 through a duct 17. FIG. 1E showsan embodiment of the invention in which the wire water removed from awire section 300 of a paper machine or equivalent is passed along a duct10 into a cyclone 25 and further into a deaeration tank 11. The purposeof the cyclone is to remove air from wire water already before thedeaeration tank 11 proper. The deaeration tank 11 having no overflowcomprises a discharge duct 12 for air-free wire water, which dischargeduct 12 ends in an overflow 14 at a lower level in a tank 13 inaccordance with the invention. As axonometrically shown in the figure,the end of the discharge duct 12 is arranged to lead through a side wall13′ of the tank 13, and the end of the discharge duct 12 opens to a freeair space and a normal air pressure inside the tank 13. Thus, in theduct 12, a constant pressure prevails in the air-free wire water. Asillustrated in the figure, tubes 15 a ₃ or ducts open from the branchduct 12 in a lower position in the vertical plane with respect to theoverflow 14, through which tubes or ducts dilution water is passed, forexample, to serve as headbox dilution water or as stock dilution water.In addition, as illustrated in the figure, thick stock m is passedthrough a duct 17 a ₃ to the discharge duct 12 and, through a branchduct 15 a ₂ situated on the opposite side, a mixture of wire water andthick stock m is passed from the duct 12 to the discharge duct 15 a ₂and further to a feed pump P₂, which feeds stock to a centrifugalcleaning plant 200. The mixture of wire water and stock is passed fromthe centrifugal cleaning plant 200 to a branch duct 15 a ₂′, which isconnected with a branch duct 15 a ₁ branching from the discharge duct12. The combined flow of stock and wire water is passed along the duct15 a ₁ to the suction side of a headbox feed pump P₁ and further througha machine screen 18 to a stock inlet header J of a headbox 100 of thepaper machine. Additives/fillers or dilution liquid can be added throughducts 17 a ₁ and 17 a ₂ to the stock to be fed to the headbox. A duct 16opens from the lower part of the tank 13 for passing wire water to thelong circulation.

[0023] A duct f leads to the deaeration tank 11 from a vacuum pump P₀ orequivalent, by means of which vacuum is drawn into the interior space ofthe tank 11 and air released from wire water is removed from the tankspace D′. In the deaeration tank, air is removed only from wire water.After that, the wire water is used at different locations, among otherthings, for dilution of stock. A cyclone-shaped device 25 is used beforethe deaeration tank, a centrifugal field being produced in said deviceto separate air in the form of bubbles, and there may be severalcyclone-shaped devices 25 for different water fractions of the wiresection A flow duct is also used before the deaeration tank, the flowcontaining more air being separated from the upper part of said flowduct.

1. An apparatus for passing stock to a headbox (100) of a paper machineor equivalent, which apparatus comprises a deaeration tank (11) which isprovided with vacuum by means of a vacuum pump (P₀) or another device,and the deaeration tank (11) comprises an inlet duct (10) through whichit is supplied with wire water, characterized in that the deaerationtank (11) comprises a discharge duct (12) and, at the discharge endthereof an overflow (14) for the wire water in the discharge duct (12),said overflow (14) of the discharge duct (12) being located below thedeaeration tank (11) and opening to a free air space, and that thedischarge duct (12) includes a branch duct (15 a ₁) for a flow which ispassed to the headbox (100).
 2. An apparatus according to the precedingclaim, characterized in that the end of the discharge duct (12) formsthe overflow (14).
 3. An apparatus according to the preceding claim,characterized in that the discharge duct (12) is arranged to lead firstdownwards and in its end area upwards in the vertical direction, inwhich connection the end of the discharge duct (12) opens upwards to afree air space and to a normal air pressure.
 4. An apparatus accordingto the preceding claim, characterized in that the discharge duct (12) isarranged to lead through a wall (13′) of a stock and/or wire water tank(13) and it opens upwards inside the tank (13).
 5. An apparatusaccording to the preceding claim, characterized in that the end of thedischarge duct (12) opens in the tank (13) in a space above the liquidsurface in the tank (13).
 6. An apparatus according to claim 1,characterized in that the end of the discharge duct (12) is connected toa tank (13) and that the tank (13) comprises an overflow (14) for thewire water in the discharge duct (12).
 7. An apparatus according toclaim 1, characterized in that the lower end of the discharge duct (12)is arranged to lead inside a tank (13) and it opens to a free airspace/air pressure and that, in addition, the tank (13) comprises aseparate overflow (140), and that the tank (13) comprises a duct (10)leading to the deaeration tank (11), and that said duct comprises a pump4) by which wire water is pumped into the deaeration tank (11).
 8. Anapparatus according to claim 1, characterized in that a branch duct (15a ₁) is connected to the discharge duct (12) for passing-thick stock (m)and wire water to the suction side of a headbox feed pump (P₂), stockbeing passed from the headbox feed pump (P₂) to the headbox (100) of thepaper machine or equivalent.
 9. An apparatus according to claim 1,characterized in that the discharge duct (12) comprises a branch duct(15 a ₃) for dilution liquid for passing dilution liquid into theheadbox of the paper machine to different locations in the widthdirection of the headbox in order to accomplish dilution of the stock.10. An apparatus according to claim 1 or 8, characterized in that thedischarge duct (12) comprises a branch duct (15 a ₂), virgin stock (m)being added to the wire water in said branch duct and passed to thesuction side of a feed pump (P₃) and further to a centrifugal cleaningplant (200) and from the centrifugal cleaning plant (200) to the branchduct (15 a ₁) and into the headbox (100).
 11. An apparatus according toclaim 1, characterized in that, before the deaeration tank (11), acyclone-shaped device (25) is used in which a centrifugal field isproduced to separate air in the form of bubbles, and there may beseveral cyclone-shaped devices (25) for different water fractions of thewire section.
 12. An apparatus according to claim 1, characterized inthat a flow duct is used before the deaeration tank, the flow containingmore air being separated from the upper part of said flow duct.